The main thing that stops bad cells from proliferating isn't telomeres, but our own immune system. NK cells (natural killers) are very efficient in terminating everything suspicious with extreme prejudice, so to say.

Successful cancer is the one that learns how to look innocently, at least to the immune cells patrolling the body.

Successful immunological treatment (actually using mRNA vaccines, that is what Moderna and BioNTech were originally about; Covid vaccines are only an adaptation) teaches the immune system to recognize the bad cells again based on their mutations. It will then jump into action and control the cancer growth.